1. Field of the Invention
The invention relates to a method of manufacturing a flat electrical cable used e.g. for wiring public utility apparatuses, office automation apparatuses, electronic parts mounted in vehicles, and the like. In particular, the invention concerns devices used for the manufacture of such a flat cable. The invention further concerns a flat cable produced by such a manufacturing method or device.
2. Description of Background Information
FIGS. 1A and 1B show how the prior art flat cables were manufactured in the past. According to a known method, a pair of insulator films 100 is prepared, so that each includes a base film 101 and an adhesive layer 102, the adhesive layers facing each other. A plurality of rectangular conductors 103 are then arranged parallel to one another, and are flanked by the pair of insulator sheets 100 over their length. Subsequently, the insulator sheets 100 are thermally bonded e.g. by heated rollers.
In the above manufacturing method, the adhesive layers 102 of the insulator sheets 100 have to be sufficiently thick for each of the rectangular conductors 103 to be deeply embedded in the adhesive layers 102, with the insulator sheets 100 firmly adhered and bonded. To obtain such a result, the base film 101 and adhesive layer 102 of each insulator sheet 100 must usually have an approximate thickness of 50 xcexcm and 60 xcexcm, respectively.
In such a construction, because the adhesive layers 102 must have substantial thickness, material costs are relatively high. In addition, as the adhesive layers 102 usually employ combustible agents, a flat cable including such a thick adhesive layer 102 raises the problem of inflammability.
Another flat cable manufacturing method is disclosed in Japanese Patent Application published under No.2000-502 833, according to which the rectangular conductors are initially arranged parallel to one another and sandwiched by two insulator sheets containing no adhesive layer, the insulator sheets being bonded to each other by ultrasonic welding.
However, ultrasonic welding alone does not allow the insulator sheets to be sufficiently well bonded, especially in the longitudinal zones extending between the rectangular conductors contained in the flat cable. When the insulator sheets are only loosely adhered, they may form air gaps therebetween, into which water or moisture from condensation may penetrate. Such a phenomenon creates a high risk of short-circuiting between the rectangular conductors.
In parallel, research is currently underway for a flat cable which requires a smaller space and weighs less. In this case too, the flat cable is manufactured by providing two insulator sheets, and interposing therebetween a number of mutually parallel rectangular conductor elements over the length of the sheets. According to one of the manufacturing methods, the insulator sheets flanking the conductor elements are bonded by ultrasonic welding.
According to one of the known ultrasonic welding methods, the side edges of insulator sheets which are positioned outside the parallel array of conductor elements are bonded intermittently by ultrasonic welding along the length direction of the sheets. However, in such a flat cable, electrically conductive material such as water tends to penetrate through non-bonded portions, thereby causing short circuits between the conductor elements.
The present invention aims at solving the problem caused by such an accident-prone flat cable, and provides a method for its manufacture, according to which the insulator sheets are intimately and firmly adhered through an adhesive layer, and, moreover, in which the adhesive layer can be made as thin as possible.
The invention also aims to provide a system for producing a flat cable, in which newly conceived ultrasonic welding units are used.
Further, the invention aims at manufacturing a flat cable, in which the conductor elements are efficiently prevented from short-circuiting.
To this end, there is provided a method of manufacturing a flat cable having a length and a width, the flat cable including first and second insulator sheets, and at least one adhesive layer interposed therebetween. The flat cable further includes a plurality of conductor elements arranged in parallel relation to one another over the length of the first and second insulator sheets. The method includes providing the adhesive layer on at least the first insulator sheet so as to face the second insulator sheet, interposing the conductor elements between the adhesive layer and the second insulator sheets, provisionally adhering the first and second insulator sheets including the conductor elements through the adhesive layer by means of heat pressing force exerted from outside the first and second insulator sheets, and bonding, by ultrasonic welding, the first and second insulator sheets through the zones extending along the length thereof and located outside the loci where the conductor elements are arranged.
Preferably, the adhesive-layer providing includes providing a first insulator sheet having a thickness of about 12 to about 300 xcexcm and an adhesive layer having a thickness of about 1 to about 3 xcexcm.
Preferably yet, the adhesive-layer providing includes providing an adhesive layer formed of the same type of material as that of the first insulator sheet, the adhesive layer containing no halogen-based flame-retardant.
Typically, the adhesive-layer providing includes providing the first and the second insulator sheets, so that one of them is at least about 1.5 times thicker than the other.
The invention also proposes a system for manufacturing a flat cable having a length and a width, the flat cable including first and second insulator sheets, and at least one adhesive layer interposed therebetween. The flat cable further includes a plurality of conductor elements arranged in parallel relation to one another over the length of the first and second insulator sheets. The system includes, along a production flow line from upstream to downstream, an adhesive application unit that provides the adhesive layer on at least the first insulator sheet so as to face the second insulator sheet, a conductor feed unit that interposes the conductor elements between the adhesive layer and the second insulator sheets, a provisional adhering unit that provisionally adheres the first and second insulator sheets including the conductor elements through the adhesive layer by a heat pressing force exerted from outside the first and second insulator sheets, and an ultrasonic welding unit that bonds the first and second insulator sheets through the zones extending along the length thereof and located outside the loci where the conductor elements are arranged.
Preferably, the above ultrasonic welding unit includes a horn that imparts ultrasonic oscillations, and an anvil located in opposition to the horn. The horn includes an oscillation-imparting portion, with which one of the first and second insulator sheets is placed into contact, and the anvil has a generally cylindrical form and includes an axis arranged perpendicularly to the production flow line, the anvil and being freely rotatable around the axis. The generally cylindrical body includes an outer cylindrical face including an appropriate number of arrays of protrusions, the protrusions being aligned in the circumferential direction of the outer cylindrical face and extending at a given interval therealong, such that the arrays of aligned protrusions can be placed into contact with the other of the first and second insulator sheets at both sides of each of the conductor elements. Preferably yet, the ultrasonic welding unit includes a horn that imparts ultrasonic oscillations, and an anvil located in opposition to the horn. The horn includes a plane, with which one of the first and second insulator sheets is placed into contact over the width hereof. The anvil has a generally cylindrical form that can rotate freely around an axis, the axis being provided perpendicular to the production flow line, and the anvil includes an outer circular face including an appropriate number of circular ribs continuously extending in the circumferential direction thereof, such that the circular ribs can be placed into contact with the other of the first and second insulator sheets at both sides of each of the conductor elements.
Each of the circular ribs may have an alternating broad and narrow width.
Typically, the ultrasonic welding unit includes first and second horns that impart ultrasonic oscillations, located respectively upstream and downstream on the production flow line at a given distance, and further includes corresponding first and second anvils located in opposition to the first and second horns. The first and second horns respectively include an oscillation-imparting generally cylindrical body, the generally cylindrical body having an axis arranged perpendicularly to the production flow line and being freely rotatable around the axis. Each of the oscillation-imparting cylindrical bodies has an outer cylindrical face including an appropriate number of arrays of protrusions aligned in the circumferential direction thereof and extending at a given interval therealong, such that the arrays of protrusions can be placed into contact with one of the first and second insulator sheets at both sides of each of the conductor elements. The first and second anvils are fixedly positioned and respectively include a plane, with which the other of the first and second insulator sheets is placed into contact, so that the first horn and anvil can form, by ultrasonic welding, a first series of intermittent bonded portions with a given interval therebetween, and the second horn and anvil then form a second series of bonded portions in the given interval.
Alternatively, the ultrasonic welding unit may include a horn that imparts ultrasonic oscillations, and an anvil located in opposition to the horn, the horn and anvil including respective planes opposing each other. The opposing planes of the horn and anvil include, respectively, an appropriate number of arrays, respectively formed of protrusions and recesses, each of the protrusions having the same gauge as each of the recesses along the length of the first and second insulator sheets, such that the arrays of the opposing face of the horn and those of the anvil can be placed into contact respectively with the first and second insulator sheets at both sides of each of the conductor elements. Thus, the first and second insulator sheets can be flanked by the arrays formed of protrusions and recesses of the horn and of the anvil, and subjected to a first ultrasonic welding, thereby forming intermittent first bonded portions, and the first and second insulator sheets can be moved by a distance equivalent to the gauge, and the first and second insulator sheets can be further subjected to a second ultrasonic welding, thereby forming second bonded portions that link the first bonded portions.
Alternatively yet, the ultrasonic welding unit may include a horn that imparts ultrasonic oscillations, and an anvil located in opposition to said horn. One of the horn and the anvil has a plane with a length which extends along the length of the first and second insulator sheets, while the other includes a cylindrical body having an axis arranged perpendicularly to the production flow line and being freely rotatable around the axis, the cylindrical body being movable back and forth along the production flow line. Additionally, the cylindrical body has an outer cylindrical face including an appropriate number of circular ribs extending in the circumferential direction of the outer cylindrical face, so that the circular ribs can be placed into contact with the first and second insulator sheets at both sides of each of the conductor elements.
As a variant, each of the circular ribs may have an alternating broad and narrow width, or each of the circular ribs may be provided with recesses.
A further object of the invention is to provide a flat cable including first and second insulator sheets having a length and a width, and containing a plurality of conductor elements arranged in parallel relation to one another over the length of the first and second insulator sheets. The flat cable further includes an adhesive layer which bonds the first insulator sheet, the second insulator sheet and the conductor elements interposed therebetween, by means of heat pressing.
In the above flat cable, the first and second insulator sheets are bonded at the zones extending along the length thereof and are located outside the loci where the conductor elements are arranged.
As a variant, each of the zones may be bonded in an intermittent way, or in a continuous way.
Preferably, the adhesive layer includes the same type of material as that of the first insulator sheet, the adhesive layer containing no halogen-based flame-retardant.
Typically, one of the first insulator sheet and the second insulator sheet is at least 1.5 times thicker than the other.